(1) Field of the Invention
The present invention relates to an image coding method, an image decoding method, and the like, for video.
(2) Description of the Related Art
In an image coding method for coding video, an amount of information is generally compressed using redundancy in a spatial direction and a temporal direction held by the video. In general, conversion into a frequency region is used as the method of using the redundancy in the spatial direction, and an inter picture prediction (hereinafter referred to as inter prediction) coding is used as the method of using the redundancy in the temporal direction. When a picture is coded in the inter prediction coding, a coded picture that precedes or follows a current picture to be coded in display order is used as a reference picture. A motion vector is derived by performing motion estimation on the current picture with respect to the reference picture, and a difference is calculated between image data of the current picture and a predictive picture obtained by performing the motion compensation based on the motion vector, thereby removing the redundancy in the temporal direction.
With the image coding standard called H.264 which has already been standardized, three types of pictures; that is, I-picture, B-picture, and P-picture, are used for compression of the amount of information. The I-picture is a picture on which the inter prediction coding is not performed, in other words, a picture on which intra picture prediction (hereinafter referred to as intra prediction) is performed. The P-picture is a picture on which the inter prediction coding is performed with reference to a coded picture that precedes or follows a current picture to be coded in display order. The B-picture is a picture on which the inter prediction coding is performed with reference to two coded pictures that precede or follow the current picture in display order In addition, the I-picture and the P-picture include a switching slice and the like (SI slice, SP slice) for switching between streams, and the like.
With the image coding method and the image decoding method that conform to the existing H.264 standard, these methods are required to be applicable to all of the reference relations that can be conceived by the definition of the pictures of these types. When decoding a P-picture, for example, it is required that a picture that follows in the display order is allowed to be referred to. In addition, when two motion vectors are present for the B-picture, the directions of these two motion vectors may be forward or backward. In addition, pictures which are different for each block in a slice may be referred to. In order to correspond to the flexibility of such a reference structure, execution of some processes are required for the image coding method and the image decoding method in H.264 (see, for example, Non Patent Literature 1: ITU-TH. 26403/2010).